Medical practice often requires the measurement of subtle pressure changes inside the body, and many different means have been devised to determine these values. Particular interest has been devoted to the determination of pressures inside such structures as the urethra and the coronary arteries.
When fluid filled catheters are used to convey pressure to sensors located outside the body, a variety of errors result from extraneous parameters such as air temperature, catheter length, atmospheric pressure, and patient position. The recent development of miniaturized strain gauged elements, where the entire sensor can be inserted into the body cavity, has helped to control some of these variables. However, since most sensors rely on a single instrumented surface, they are peculiarly sensitive to rotational effects which cause the reported pressure to vary widely from the true circumferential pressure due to localized blocking of all or a part of the element. Brooks describes a method of measuring circumferential pressure in U.S. Pat. No. 4,711,249, but the substantial blocking of the lumen of the supporting tube by the transverse location of the sensing elements limits its effectiveness in certain applications.
A particular area of interest for measurement of circumferential pressures is in the evaluation of the human urinary sphincter and the quantification of its effectiveness in controlling the flow or urine. In evaluating the urinary sphincter, a series of pressure measurements must be taken along the length of the urethra where it is surrounded by the sphincter muscle. At the same time, the fluid pressure inside the bladder must be measured so that the ability of the sphincter to close the urethra to the passage of the urine can be determined. Prior technology for obtaining a urethral pressure profile involves the insertion of a catheter incorporating two pressure sensors, the first sensor measuring bladder pressure and the second measuring pressure at a fixed location relative to the first. The profile is obtained by moving the catheter at a controlled rate and recording the pressures.
The accuracy of the profile is facilitated by the ability to withdraw urine from the bladder or introduce fluid into it so as to control bladder distension during a series of tests. Additionally, the ability to make all measurements simultaneously is highly desirable, since transient events such as coupling or muscular exertion are not accurately reproducible for serial measurements of pressures in differing locations.
All of the foregoing functions must be performed by a urodynamic pressure measurement probe that is small enough to fit within a normal urethra without causing undue distension of the surrounding tissue, is protected from corrosive and electrically conductive urine, and is rugged, easily sterilized and electronically stable. If it is too large or irregularly shaped the device will produce false pressure readings as well as inducing discomfort in the patient being tested. If it generates discomfort, its efficacy will be limited due to failure of the patient to cooperate when required to bear down, cough or more.
Therefore, an object of the present invention is to provide a sensor for the measurement of circumferential pressures inside body cavities, thereby removing the measurement errors introduced by sensors which only measure pressure at a discrete point or aperture.
A further object of the present invention is to provide a urodynamic pressure measuring probe in the form of a thin catheter with multiple circumferential pressure sensors along its length, thereby allowing a pressure profile to be obtained without moving the catheter.
A further object of the present invention is to provide a urodynamic pressure measuring probe which is compact, lightweight and durable so that it may be connected to a telemetry system for monitoring patients during normal activities.